1. Field of the Invention
The present invention relates to a photoelectric conversion device having a photoelectric charge storage region whose potential is controlled by a capacitor.
2. Related Background Art
FIG. 1A is a plan view showing the pattern of a photoelectric conversion device described in U.S. Pat. No. 4,686,554, based on Japanese Patent Applications Nos. 120751/1983 to 120757/1983, FIG. 1B is a sectional view along line A-A' of FIG. 1A, and FIG. 1C is an equivalent circuit of a photosensor cell of the photoelectric conversion device.
Each photosensor cell of the photoelectric conversion device is constructed of insulation films 2 and 3 such as SiO.sub.2 formed on an n-type silicon substrate 1; an isolation region 4 for electrically isolating each photosensor cell; an n.sup.- -region 5 of low impurity concentration formed by the epitaxy method or the like; a p-region 6 serving as a bipolar transistor base; an n.sup.+ -region 7 serving as an emitter; an interconnection 8 made of conductive material such as A1 and connected to the n.sup.+ -region 7 at a contact 19 for reading a signal to the external circuit; a capacitor electrode 9 facing the p-region 6 with the insulation film 3 interposed therebetween for controlling the potential of the p-region 6 under floating condition; an interconnection 10 connected to the capacitor electrode 9; an n.sup.+ -region 11 formed on the back of the substrate 1 to provide ohmic contact the substrate 1; and an electrode 12 for supplying a collector potential of the transistor.
Referring to the equivalent circuit shown in FIG. 1C, the capacitor Cox 13 is made of a MOS structure having the electrode 9, insulation film 3 and p-region 6. The bipolar transistor 14 is constructed of the emitter n -region 7, base p-region 6, collector n.sup.- -region 5 and substrate 1.
The fundamental operations include storage, read and refresh operations.
First, the p-base region 6 at a negative potential is floated so that the holes of light-induced electron/hole pairs are stored in the p-base region 6 (storage operation).
Next, a positive read pulse is applied to the capacitor electrode 9 to raise the potential of the p-base region 6 and forward bias the emitter-base junction so that a storage voltage generated by the stored holes is read to the emitter electrode 8 in a floating condition (read operation).
The holes stored in the p-base region 6 are moved toward the emitter by grounding the emitter electrode 8 and applying a positive refresh pulse to the capacitor electrode 9 (refresh operation). After the stored holes are removed from the emitter, the p-base region 6 takes an initial negative potential at the trailing edge of the positive refresh pulse. The above storage, read and refresh operations are repeated thereafter.
The conventional photoelectric conversion device has the emitter and capacitor electrodes 8 and 9 formed on the light receiving side, thereby blocking a portion of the light receiving surface. Therefore, a problem arises that the more the photosensor cell is made small, the more the aperture size becomes small.